DS1227 Datasheet, PDF (17/20 Pages) Dallas Semiconductor – KickStarter Chip

DS1227
The schematic in Figure 1 demonstrates four kickstart
stimuli. They are real time clock alarm, RS232 incoming
data, a sensor input, and a user switch. During the low
power standby prior to kickstart, V_CCO from the DS5000
provides battery power to the RAM and real time clock
from the button cell. V_DCO supplies the RS232 trans-
ceiver. While operating on battery power, the RTC can
still issue alarms. If a time of day alarm is programmed,
INTA will be taken low by the RTC when the alarm oc-
curs. Th is is connected to ON1, and issues a Kickstart.
Incoming RS232 activity will allow the transceiver to
Kickstart the DS1227. Following the initial interrupt, all
additional RS232 data is passed through the RXIN/
RXOUT pins of the DS1227 to the DS5000 without fur-
ther action. In this way, the instrument can collect a table
of data and dump it to a PC for analysis when necessary.
Since the instrument will kickstart when it detects
RS232 communication, it is unnecessary for an opera-
tor to take further action. Enough time should be allowed
for the DS5000 to complete a power-on reset before
sending meaningful data.
Two additional methods of kickstarting are illustrated.
One method involves the use of a sampled sensor. A pe-
riodic pulse (the watchdog alarm) from the DS1283
causes V_CC04 to be applied to the LED. For example,
this might occur every 250 ms. It remains on for the time
it takes to charge the capacitor on Mode4 to 1/2 V_DCO
(1.5V). In this example, the on period is approximately
75 µsec. Just prior to removing V_CC04, the sense line is
sampled. If the LED light path to the photodiode is
blocked, the sense line will be high and the system will
be kickstarted. If the light path is clear, the sense line will
be low, and nothing will happen. This facilitates check-
ing for the presence of an ID card in a reader. In the other
method a user switch, which is momentarily closed, will
start the system. This is tied to ON1 in a wired-OR con-
figuration. All of the above kickstart stimuli cause the
boost regulator to raise V_DCO and turn on V_CC01. In
summary, the four kickstart stimuli are:
1) Time of Day Alarm - INTA goes low and Kickstarts
V_CC01.
2) RS232 Activity - Powers up V_CC01 and routes all
RS232 straight through to the DS5000.
3) INTB goes low periodically, V_CC04 turns on, and the
sense line is sampled. If high, a kickstart occurs. If
low, no action.
4) A user switch momentarily pulls ON1 low and kick
starts.
Although the user switch is easily implemented, it may
be unnecessary. By allowing the instrument to power up
and determine the cause of the Kickstart, it is possible to
achieve buttonless operation in many applications. Au-
tomatic response allows the instrument to function au-
tonomously and save power by turning off unused cir-
cuits.
Once the DS5000 receives power, it must read the INT/
ACK line (tied to INT0). A power-on condition causes
this signal to be low. The DS5000 port pin should then
acknowledge power up by driving this line high. This
recognizes the interrupt and enables the kickstarter for
further activity. The DS5000 may now turn on auxiliary
loads V_CC02 and V_CC03 using ON/OFF 2 and 3 (tied to
any port pins). These auxiliary supplies may supply cir-
cuits which are not always necessary (e.g. an A/D con-
verter). Peripheral circuits remain powered down until
needed. After an operation is complete, the DS5000 can
turn off the auxiliary circuits. When processing of a task
is complete, it may turn itself off using OFF1. An applica-
tion may require that an auxiliary circuit remain on when
the microcontroller is off. This might occur with an LCD
display or dual slope A/D converter. Since the dual slope
A/D takes a relatively long period to convert (40-50 mS),
the microcontroller may be powered down while waiting.
Since the INT/ACK line is tied to INT0, additional kick-
start stimuli which occur while VCC01 is on will cause
the DS5000 to receive an interrupt. This allows the
DS5000 to take action for specific conditions.
Precautions against excessive current drain are taken
in this application. For example, the data input to the
DS1275 RS232 transceiver is tri-stated when VCC01 is
off. This is necessary to prevent a high signal from driv-
ing the RS232 bus and consuming power while the
DS5000 is off. Similar precautions should be taken by
the user in designing systems with switched power sup-
plies.
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